Magnetic materials undergo some elongation or contraction when subjected an external magnetic field to that. This phenomenon is known as magnetostriction. Magnetostriction is utilized in a variety of device applications, for example, displacement controlling or driving actuators, magnetostrictive oscillators for generating ultrasonic waves, ultrasonic delay lines, ultrasonic filters, variable frequency resonators, and various sensors.
Magnetostrictive materials are essentially required to have large magnetostrains. Known magnetostrictive materials having large magnetostrains include, for example, (i) alloys of iron with rare earth elements such as Tb, Sm, Dy, Ho, Er, and Tm as disclosed in U.S. Pat. Nos. 4,375,372, 4,152,178, 3,949,351, and 4,308,474, (ii) alloys of an iron group element and Mn with Tb and Sm as disclosed in U.S. Pat. No. 4,378,258, and (iii) magnetostrictive materials consisting of Fe, A1, Co, and at least one element selected from the group consisting of Ti, V, Cr, Mn, Ni, Cu, Nb, Mo, Ta, W, C, Si, Ge, Sn, B, In, La, Ce, Pr, Nd, Sm, Gd, Tb, Eu, Dy, Ho, Er, Yb, Lu, and Tm and magnetostrictive materials consisting of Tb, Dy, Ho, Fe, and at least one element selected from the groupconsisting of Ti, V, Cr, Mn, Co, Ni, Cu, Nb, Mo, Ta, W, C, Si, Ge, Sn, B, In, La, Ce, Pr, Nd, Sm, Gd, Eu, Er, Yb, Lu, and Tm as disclosed in JP-A 64798/1978. These magnetostrictive materials are intermetallic compounds of Fe and rare earth element R, which are known as RFe.sub.2 Laves phase intermetallic compounds, optionally having another element such as a transition metal added thereto. They are called giant-magnetostrictive materials since they have saturation magnetostrictions .lambda.s which are at least ten times greater than those of conventional nickel and ferrite base magnetostrictive materials.
Among the RFe.sub.2 Laves phase intermetallic compounds, TbFe.sub.2 produce large magnetostrains under a strong external magnetic field, for example, of about 20 to 25 kOe, but insufficient magnetostrainS under a weak magnetic field. Modified compounds wherein Tb is partially replaced by Dy, that is, of the formula (Tb,Dy)Fe.sub.2 are widely used as magnetostrictive materials capable of producing superior magnetostriction with low magnetic field strength. In particular, magnetostrictive material having Tb.sub.0.3 Dy.sub.0.7 Fe.sub.2.0 crystals is most practical and widespread since it has minimal crystalline magnetic anisotropy at room temperature and a large magnetostriction.
In order to further increase the magnetostriction of these giant-magnetostrictive materials, it is effective to impart anisotropyby orienting grain axes lying in a larger magnetostriction direction. RFe.sub.2 Laves phase intermetallic compound grains have larger magnetostriction in the [111] axis direction. Particularly in the above-mentioned Tb.sub.0.3 Dy.sub.0.7 Fe.sub.2.0 grains, the [111] axis is also an axis of easy magnetization.
Single crystal growth techniques are effective for grain orientation. For example, U.S. Pat. No. 4,308,474 discloses a Bridgman method. However, the single crystal growth techniques are very low in productivity and can produce only a limited shape of product as compared with powder metallurgy. Although orientation of [111] axis is described in the patent cited herein, later research works revealed that in the Bridgman method, orientation occurs along [11-2] axis rather than [111] axis. Since [11-2] axis is offset about 19.degree. from [111] axis, the magnetostriction associated with [11-2] axis orientation is reduced to the magnetostriction associated with [111] axis orientation multiplied by cos19.degree. . The patent cited herein describes an example wherein an isotropic polycrystalline alloy prepared by arc melting is subject to grain orientation by a perpendicular zone melt method. Like the single crystal growth technique, the anisotropic orientation by the zone melt method also suffers from very low productivity and a limited freedom of shape, with orientation occurring along [11-2] axis.
On the other hand, the powder metallurgy enables low cost manufacture. U.S. Pat. No. 4,152,178 discloses that an anisotropic material with [111] axis alignment is obtained by compacting a powder of particles having Tb.sub.0.3 Dy.sub.0.7 Fe.sub.2.0 grains in a magnetic field, followed by sintering. However, because of insufficient magnetic anisotropy of Tb.sub.0.3 Dy.sub.0.7 Fe.sub.2.0 grains, a strong magnetic field must be applied in order to align particles. Sometimes particles are not aligned, leaving a problem with respect to the practice of this method.
JP-A 180943/1989 discloses a method involving the steps of separately pulverizing TbFe.sub.2 and DyFe.sub.2, mixing them, compacting the mixture in a magnetic field, and sintering the compact. It is described therein that both TbFe.sub.2 and DyFe.sub.2 are easy to align because of greater magnetic anisotropy and convert to a low magnetic anisotropy composition due to diffusion during sintering, resulting in an enhanced response to a magnetic field. However, since the axis of easy magnetization is [111] axis for TbFe.sub.2 and [100] axis for DyFe.sub.2, an anisotropic material with [111] axis orientation could not be produced by this method in our follow-up test which will be described later.